The present invention relates to a method and apparatus for spectral analysis of images, and particularly for analyzing an optical image of a scene to determine the spectral intensity of each pixel thereof. A spectrometer is an apparatus designed to accept light, to separate (disperse) it into its component wavelengths, and detect the spectrum. An imaging spectrometer is one which collects incident light from a scene and analyzes it to determine the spectral intensity of each pixel thereof.
The conventional imaging spectrometer includes a slit in the imaging plane for scanning the scene to be analyzed and focusing the scanned light on an array of detectors. When a two-dimensional detector array is used, one of the dimensions of the array is used to sample the different wavelengths associated with a single pixel, while the field of view is covered by a one-dimensional scanner and the remaining dimension of the array. When a one-dimensional detector array is used, the field of view is scanned mechanically in two directions, and all the detectors are used at any given time only to sample the different wavelengths of a single pixel. In both cases, the slit in the image plane ensures that each detector sees only the contribution of a single pixel at a single wavelength at any time; otherwise, it would be impossible to separate the spectra of each pixel.
However, conventional slit-type imaging spectrometers suffer from the disadvantage that most of the pixels of one frame are not measured at any given time, even though the fore optics of the spectrometer actually collects incident light from all of them simultaneously. Thus, the conventional slit-type technique is wasteful of the available information since, except for one wavelength, most of the radiation emitted by the measured pixel at any given time and reaching a particular detector are rejected. The result is that either a relatively large measurement time is required to obtain the necessary information with a given signal-to-noise ratio, or the signal-to-noise ratio (sensitivity) is substantially reduced for a given measurement time.
An object of the present invention is to provide a novel method and apparatus for spectral analysis of images which have advantages in the above respects.
More particularly, an object of the invention is to provide a method and apparatus for spectral analysis of images which better utilizes all the information available from the collected incident light of the image to substantially decrease the required frame time and/or to substantially increase the signal-to-noise ratio, as compared to the conventional "slit" type of imaging spectrometer.